Solutrean and Magdalenian ferruginous rocks heat-treatment: accidental and/or deliberate action?
Résumé
Heating of prehistoric coloring materials can induce radical changes in color indicative of structuralmatter transformation. For instance, the structure of the yellow iron oxide-rich mineral, goethite,changes into the red iron oxide-rich mineral, hematite, when it is heated to around 250-300°C. For along time, heating has been thought to be the reason for the high frequencies of red rocks used in campsites and the red pigments in rock art paintings. However, records of heat-treatment of coloring materialsare usually not well documented; the contextual information is not clear enough to confirm intentionalheating. Two Solutrean camp sites (the flint workshop Les Maîtreaux and the hunting site CombeSauniere I) and one middle Magdalenian cave with rock art (Grotte Blanchard, La Garenne) allow us tostudy the heating process of ferruginous rocks. All three sites, which have been excavated relativelyrecently, have well-defined archaeological records and strong associations between the ferruginous rocksand other artifacts. With the use of X-ray diffraction and electron m-diffraction for identifying structuralmodification and SEM-FEG and TEM-FEG for detecting dehydration nano-pores, we have strong evidencefor intentional heat-treatment of yellow goethite-rich materials in two archaeological contexts and onesite for unintentional heating, where rocks were only partially transformed. Intentional heating to obtainred hematite from primary goethite would have required ingenious methods of temperature control infireplace settings and purpose-built ground ovens.
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